JP2000311694A - Laminated electrode for electrochemical battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery having a high catalyst utilization factor and improved in the movement of produced water. SOLUTION: This laminated electrode is an electrode structural body including a collector sheet, a first electrode layer and a second electrode layer, and the first electrode layer is located between the collector sheet and the second electrode layer; the first layer includes a first group of carbon particles, and the second layer includes a second group of carbon particles; the first layer includes no catalyst or includes a first group of ultra-fine crushed catalyst particles; the second layer includes a second group of ultra-fine crushed catalyst particles; however, the weight ratio of the catalyst particles to carbon particles of the first layer is less than that of the second layer. In this case, for instance, the density per cm3 of the first group of the carbon particles is set to 1 gram or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an electrochemical cell (elevation cell).
related to electrodes for use in ctrochemical cells).

[0002]

BACKGROUND OF THE INVENTION Electrochemical cells are desirable for a variety of applications, particularly when operating as fuel cells. Fuel cells have been proposed for many applications, including electric motors, to replace internal combustion engines. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or a proton exchange membrane (PEM) for ion exchange between the cathode and anode. Gas and liquid fuels are convenient in fuel cells. Examples of these include hydrogen and methanol, with hydrogen being preferred. Hydrogen is supplied to the anode of the fuel cell. Oxygen (as air) is the oxidant of the battery and is supplied to the cathode of the battery. To facilitate distribution of fuel on the membrane surface facing the fuel supply electrode, the electrode is formed from a porous conductive material (eg, knitted graphite, graphitized sheet, or carbon paper). Exemplary fuel cells are described in U.S. Patent Nos. 5,272,017 and 5,316,871 (Swathirajan et al.).

[0003] Important characteristics of a fuel cell include a reaction surface where an electrochemical reaction occurs, a catalyst that catalyzes such a reaction,
Ionic conductive media and mass transport media
media). The cost of power generated by the fuel cell depends in part on the cost of the catalyst. Due to the relatively low utilization of expensive metal catalysts in conventional electrodes, the power cost generated by a fuel cell is much higher than another competitive power generation cost. However, because hydrogen is environmentally acceptable and hydrogen fuel cells are efficient, power generated from hydrogen-based fuel cells is preferred. Therefore, there is a need for improved catalyst utilization in fuel cell assemblies to make fuel cells more attractive for power generation. It is also desired to improve the movement of water generated by the fuel cell and the gas dispersion of the reactants.

[0004]

In one aspect, an electrode structure is provided that includes a current collector sheet, a first electrode layer, and a second electrode layer. The first electrode layer is between the current collector sheet and the second electrode layer. The first layer includes a first group of carbon particles, and the second layer includes a second group of carbon particles. The first layer contains no catalyst (unc
atalyzed) or comprises a first group of micronized catalyst particles (catalyst)
lyzed), the second layer comprises a second group of micronized catalyst particles. The weight ratio of catalyst particles to carbon particles in the first layer is less than the weight ratio of the second layer.

In one embodiment, each group of carbon particles comprises a plurality of carbon particles having internal and external surfaces defining a plurality of pores (plethora of pores) within and between the carbon particles. including. The micronized catalyst particles are supported on the interior and exterior surfaces of the carbon particles.

In another embodiment, the first layer does not contain a catalyst,
The second layer includes carbon particles having micronized catalyst particles supported on the inner and outer surfaces of the carbon particles.

[0007] Preferably, the first group of carbon particles comprises:
It is characterized by a density of no more than 0.1 gram per cubic centimeter, which corresponds to a volume / gram of at least about 10 cubic centimeters per gram. Desirably, the second group of carbon particles is characterized by a pH in the range of about 6 to about 9. Preferably, each group of carbon particles is characterized by a pH in the range of about 6 to about 9. Desirably, the second group of carbon particles is characterized by an average pore radius of greater than 5 nanometers. Furthermore, each layer includes a proton conductive material mixed with carbon particles and catalyst particles.

[0008] Desirably, the catalyst particle loading of the second layer is less than about 0.30 mg / cm ² of electrode surface area.
Catalyst loading of the first layer is less than the catalyst loading of the second layer, preferably from about 0.15 mg / cm ² or less in order is preferably about 0.02 mg / cm ² or less in order .

[0009] In one embodiment, the second layer is about 20: 8.
It contains catalyst particles and carbon particles in a weight ratio of 0. The proton conductive material makes up 30 to 35% by weight of the second layer, and the catalyst particles and carbon particles make up the balance.

[0010] In one aspect, a method is provided for making the improved electrode structure described above for use in an electrochemical cell. The first layer of the electrode is made by forming a mixture comprising a proton-conductive material, a first group of carbon particles and most preferably catalyst particles. The mixture is applied to a current collector sheet to form a film. A second layer of the electrode is manufactured by forming a second layer on the first layer, the second layer comprising a proton-conductive material, a second group of carbon particles, and catalyst particles. including. The weight of the catalyst particles relative to the carbon particles of the second layer is greater than the weight of the first layer. According to the method of the present invention, an electrode having a very high catalyst utilization rate and a very small amount of catalyst can be produced, and therefore, it can be produced at a lower cost as compared with an electrode produced by a conventional method.

Production of an electrolyte-electrode combined structure for an electrochemical cell having an electrolyte membrane of a solid polymer proton conductive material and first and second electrodes disposed on either side of the electrolyte membrane It also provides the law. At least one electrode is formed by the method of the present invention described above. The electrode produced by the method of the present invention is placed on the first surface of the electrolyte membrane such that the second layer faces the membrane. A second electrode is placed on the opposite side of the membrane and the resulting structure is heated and compressed to adhere the electrode to the membrane. In a preferred embodiment of the method of the present invention, the assembly is exposed to a compressive load and high temperature so that some of the particles are at least partially embedded in the membrane, causing the electrodes to adhere to the membrane and thereby to the catalytic sites where the reaction occurs. To provide a continuous path of protons.

[0012] The first and second groups of carbon particles are the same or different. That is, they may have the same characteristics or may differ in at least one characteristic point. When each layer contains a catalyst, the catalyst of each layer may be the same or different.

As can be seen from the description of the electrodes, the membrane electrode assembly, and the fuel cell system described above, the present invention provides improved catalyst utilization and improved water handling (treatment).

It is an object of the present invention to provide a new electrode and a new membrane electrode assembly. It is another object of the present invention to provide a method of manufacturing an electrode and an assembly including a superior electrode. Advantageously, the membrane / electrode assembly of the present invention provides relatively high power output with unexpectedly low catalyst loading.

[0015] These and other objects, features and advantages will become apparent by reference to the preferred embodiments, the claims and the accompanying drawings.

[0016]

DETAILED DESCRIPTION In one aspect, an electrode structure is provided that includes first and second layers of electrode material and a current collector sheet. These layers together increase catalyst utilization,
Improve water handling. This stacked arrangement is particularly useful for cathodes. Before describing the electrode structure in detail, a battery including an electrode will be described first.

FIG. 1 is a pictorial illustration of an unassembled electrochemical cell 10 that includes a membrane electrolyte and electrode coupling assembly (MEA) 12 within the cell. The electrochemical cell 10 is assembled as a fuel cell. However, the invention described herein is generally applicable to electrochemical cells. The electrochemical cell 10 includes stainless steel end plates 14 and 16, graphite blocks 18 and 20 with openings 22 and 24 for facilitating gas distribution, gaskets 26 and 28, and connection portions 3 respectively.
Carbon sheet current collectors 30, 32 provided with 1, 33;
Including a membrane electrolyte and electrode assembly (MEA) 12. Graphite block, gasket and current collector, ie, 1
The two sets of 8, 26, 30 and 20, 28, 32 are referred to as gas and current transport means 36, 38, respectively. The anode connection 31 and the cathode connection 33 are used to connect to an external circuit that may include another fuel cell.

The electrochemical cell 10 operates with a gaseous reactant, one of which is a fuel supplied from a fuel supply 37 and the other is an oxidant supplied from a supply 39. Gas from the sources 37, 39 diffuses through the respective gas and current transport means 36 and 38 to the opposite side of the MEA 12.

FIG. 2 shows a schematic diagram of the assembly 12 of the present invention. Referring to FIG. 2, the porous electrode 40 forms an anode 42 on the fuel side and a cathode 44 on the oxygen side. The anode 42 is separated from the cathode by a solid polymer electrolyte (SPE) membrane 46. SPE membrane 46 provides ion transport to facilitate reactions in fuel cell 10. In one arrangement, the electrode of the present invention can be more effectively protonated by contacting the electrode with the iomonomer membrane to keep the polymer in contact essentially continuously for such proton transfer. Moves. The electrodes are preferably inserted or at least partially embedded in the membrane. Therefore, the MEA 1 of the battery 10
2 are spaced first and second facing surfaces 50, 52
And the thickness between the surfaces 50 and 52, that is, the intermediate layer region 53
And a film 46 provided with Each electrode 40, ie, anode 42 and cathode 44, is firmly adhered to membrane 46 at corresponding portions of surfaces 50, 52.

In one embodiment, each electrode 40 (anode 42, cathode 44) further comprises a first and second Teflon-coated (polytetrafluoroethylene-coated, impregnated) graphite sheet on each side of membrane 46. 80, 82 (FIG. 3). The anode active material is disposed between the first side 50 of the membrane and the first sheet 80 and the cathode active material is disposed between the second side 52 and the second sheet 8.
2 are arranged. SPE Membrane The solid polymer electrolyte (SPE) membrane 46 of the present invention is known in the art as an ionic conductive material. Such S
The PE membrane is also called a polymer electrolyte membrane (PEM). Exemplary SPE membranes are described in U.S. Patent Nos. 4,272,353, 3,134,697 and 5,211,984.

The SPE membrane or sheet is an ion exchange resin membrane. The resin contains ionic groups in its polymer structure; one ionic component is immobilized or retained by the polymer matrix, and at least one other ionic component is a mobile substituent electrostatically associated with the immobilized component. It is a possible ion. The ability of mobile ions to displace other ions under suitable conditions can render these materials ion-exchangeable.

The ion exchange resin can be manufactured by polymerizing a mixture of the components, one of which contains an ionic component. As a rough type of cation exchange and proton conductive resin, there is a so-called sulfonic acid cation exchange resin. In this sulfonic acid membrane, the cation ion exchange groups are hydrated sulfonic acid groups attached to the polymer backbone by sulfonation.

The formation of these ion exchange resins into membranes or sheets is also known in the art. The preferred type is
The entire membrane structure is a perfluorinated sulfonic acid polymer electrolyte having ion exchange properties. These membranes are commercially available, and typical examples of commercially available sulfonated perfluorocarbon, proton conductive membranes are EIDupont d under the trade name Nafion (registered trademark).
There is a commercial product of e Nemours & Co. Others have been developed by Dow Chemical. Such a proton conductive membrane has the structure: CF ₂ = CFOCF ₂ CF ₂ SO ₃ H, CF ₂ =
_{CFOCF 2 CF (CF 3) OCF} 2 SO 3 H, and -CF
₂ CF ₂ CF (ORX) CF ₂ CF _2- (where X is SO ₃
H or CO ₂ H). Nafion® is a fluoropolymer, especially a copolymer containing perfluorinated carboxylic or sulfonic acid monomer units. Nafion® polymers and polymer membranes are Nafion® polymers made from copolymers of tetrafluoroethylene with perfluorinated monomers containing sulfonic or carboxylic acid groups. For the present invention, perfluorinated sulfonic acid copolymers are preferred.

In the electrochemical fuel cell 10 exemplified by the present invention, the membrane 46 is a cation permeable proton conductive membrane having H ⁺ ions as mobile ions; the fuel gas is hydrogen (or reformate). And the oxidizing agent is oxygen or air. The overall cell reaction is the oxidation of hydrogen to water, and the respective reactions at anode 42 and cathode 44 are H ₂ = 2H + 2e (anode) and 1/2 O ₂ +2
H ⁺ + 2e = H ₂ O (cathode).

Since hydrogen is used as a fuel gas, the product of the overall cell reaction is water. Usually, the product water is
It is not acceptable at the cathode 44 which is the electrode 40 on the oxygen side. In general, water simply disappears by flowing or evaporation. However, if water is formed, means for collecting the water and transporting it out of the battery can be provided if desired. The treatment of water in the cell is important for the long-term operation of the electrochemical fuel cell.
No. 5,272,017 (hereafter '011'), which is hereby incorporated by reference in its entirety, for water treatment procedures and battery designs associated therewith.
No. 7) and No. 5,316,871 (hereinafter, referred to as No. 7).
No. '871). The invention further relates to other features such as improved water treatment during fuel cell operation, effective electrode utilization, effective proton transfer between the electrode and the membrane, and good gas diffusion. These features are enhanced, at least in part, by the improved electrode design of the present invention. Electrode The electrode of the present invention includes a current collector and an electrode active material involved in a battery reaction. Electrochemical reactions in fuel cells occur in the interface region between the proton conductive ionomer, the catalyst, the conductive carbon and the gaseous reactants. Therefore, in order to achieve high catalyst utilization, the electrodes must be designed so that the catalyst sites are in close contact with the proton exchange membrane, the gaseous reactant, and the conductive carbon.

Conventional electrodes can be prepared by conventional methods described in US Pat. Nos. 5,272,017 and 5,316,871, which are incorporated herein by reference. This is illustrated by the anode in FIGS. In such situations, catalyst-containing carbon particles
(catalyzed carbon particles) and then mixed with a proton conducting binder in a solution containing a casting solvent. The solution is applied to a Teflon-coated graphite sheet 80, the casting solvent is evaporated, the remaining layer containing the catalyst-containing carbon particles and the binder is brought into contact with the membrane and hot pressed. Here, the catalyst-containing carbon particles 60 are in close contact with and adhere to the film 46. As described herein, it is preferred that some amount of the catalyst-containing carbon particles be at least partially embedded within the membrane 46.

FIG. 4 is an enlarged pictorial view of catalyst-containing carbon particles 60 with micronized catalyst particles 62 carried on the carbon particles. The proton conductive material 64 mixes with the particles.

Although the novel electrode configuration of the present invention is described herein for use as a cathode,
It is not limited to this. It is contemplated that the novel electrodes of the present invention can be used as both anodes and cathodes, and are shown herein to be particularly advantageous when used as cathodes. The electrode of the present invention includes a current collector sheet 82, a first electrode layer 70, and a second electrode layer 72.
including. The first electrode layer 70 is between the current collector sheet 82 and the second layer 72. The first electrode layer is made of carbon particles 60.
And the second layer comprises a second group of carbon particles 60.
Group. The first and second groups of carbon particles may be the same type of carbon particles and may have the same physical properties as shown in the table. In another aspect,
The first and second groups of carbon particles are different types of carbon particles and have different characteristics. The features are defined in Table 2.

In one embodiment, the first group of carbon particles does not include a catalyst (FIG. 3). In another aspect, the first group of carbon particles forming the first layer includes a catalyst (FIG. 2). The catalyst 62 is in the form of micronized catalyst particles and is typically metal particles as described in detail below. In either embodiment, second layer 72 includes micronized catalyst particles 62.
The relative amounts of catalyst particles 62 and carbon particles 60 in the first and second layers are selected such that the weight ratio of catalyst particles to carbon particles in first layer 70 is less than the weight ratio of second layer 72. . It is clear that this condition is met if the first layer does not contain any catalyst particles and the second layer does contain the catalyst. If the catalyst particles are contained in any of the layers, the second
The weight ratio of the catalyst particles to the carbon particles in the layer is larger than the weight ratio of the first layer.

In one embodiment, the carbon particles of the first layer include a plurality of inner and outer surfaces defining a plurality of pores; the micronized catalyst particles are supported on the inner and outer surfaces of the carbon particles ( (Fig. 4). Prior to mixing with the proton conductive material 64 to form the first layer, the carbon particles 60
It is preferable to support the catalyst particles 62 on the surface.

In one embodiment, the second layer is formed in essentially the same manner as the first layer. That is, the catalyst particles are supported on the carbon particles, and then the catalyst-containing carbon particles are mixed with the proton conductive material. This mixture is then applied to the first layer to form a second layer.

The catalyst particles are preferably metallic, metallic or alloy. Most preferred are noble metal catalysts, such as platinum (Pt) and palladium (Pd).
In addition, other relatively stable metals can be used for alloying. For example, titanium, ruthenium, rhodium, tungsten, tin or molybdenum can be used.

The present invention provides a method for forming a multilayer electrode having at least first and second layers. The first layer is also called a primary layer, and the second layer is also called a main layer. The method of manufacturing the electrode structure provides (a) a current collector sheet 82; (b) a first layer 70 including a proton conductive material 64, a first group of carbon particles 60, and optionally catalyst particles 62. Formed on the sheet; and (c) a second group including the proton conductive material 64, the second group of carbon particles 60, and the catalyst particles 62.
Forming the layer 72 on the first layer. The weight of the catalyst particles relative to the carbon particles of the second layer exceeds the weight of the first layer. In one embodiment according to the above method, step (a) comprises the first group of proton conductive material, the carbon particles, and the first group of micronized catalyst particles supported on and in the carbon particles. Forming a first mixture; then performing by applying the first mixture to the surface of the current collector and forming a first film from the mixture.

In one embodiment, step (c) comprises the second group of proton conductive material, the second group of carbon particles, the second group of micronized catalyst particles supported on and in the carbon particles. Of the second mixture; then the second mixture is added to the first
This is performed by applying to the layer of

The membrane electrode assembly is manufactured by applying a multi-layer electrode and a counter-electrode to each surface of the membrane and then hot-pressing at a temperature and compressive load sufficient to adhere the electrodes to the membrane. Preferably, at least some of the electrode particles are at least partially embedded in a film that softens during high temperature heat-pressing.

In particular, the active material of anode 42 is applied to Teflon-coated graphite sheet 80. The anode active material side supported on sheet 80 is then contacted with first surface 50 of membrane 46. Cathode 44 on sheet 82
In contact with the second surface 52 of the membrane 46.
The sheets 80, 82 are hot pressed into the membrane while heating at a temperature and a compressive load for a time sufficient to soften the membrane 46 and at least partially embed at least some of the particles within the membrane, thereby forming the first and second sheets. The second electrodes 42 and 44 are formed. The embedded or intercalated particles are at least partially located on each surface of the membrane, but no particles are located below that surface or dispersed into the membrane.

The step of heating while applying pressure is about 250 to
Compressive load of about 1000 pounds per square inch and about 280
The operation is performed at a temperature between about 130 ° F. and about 320 ° F. for about 1 to about 5 minutes. A compressive load of about 500 pounds per square inch at a temperature of about 300 ° F (about 150 ° C) for about 1 to about 2 minutes has been found to be effective. The compression load may change over time. That is, a lighter load may be used for a longer time, and a heavier load may be used for a shorter time.

By embedding the electrodes in the membrane under pressure,
A continuous path of proton conductive material can be provided from one side of the membrane electrode assembly to the other. By intimately mixing the catalyst and carbon particles with the proton conductive material,
A continuous path for protons is provided at the catalytic site where the reaction takes place. In this way, a relatively optimal utilization of the catalyst particles adjacent to the membrane at the electrode is obtained.

The proportion of the proton conductive material, the catalyst particles and the carbon particles forming the anode and cathode main (second) layers is from 30 to about 70 parts of the proton conductive material based on 100 parts, The balance is catalyst particles and carbon particles. The ratio of catalyst particles to carbon particles is about 20 parts by weight or less based on 100 parts by weight, and the balance is carbon particles. The cathode primary layer (first layer) contains no catalyst or less catalyst particles. The amount is on the order of 0.02 mg / cm ² of catalyst particles. This corresponds to about 5 parts by weight of catalyst particles and 95 parts by weight of carbon particles.

In one embodiment, the cathode comprises a first layer comprising carbon particles mixed with a proton conducting material; or the first layer comprises a small amount of the order of 0.02 mg / cm ² (5 weight percent platinum). Platinum contains carbon particles containing a catalyst so that the balance is carbon. This layer typically contains 40 weight percent proton conductive material (Nafion), with the balance being on the order of 60 weight percent carbon or catalyst-containing carbon. This layer typically has a thickness of about 10 to about 13 microns.
The second layer contains 20 weight percent platinum catalyst-containing carbon particles. The weight ratio of Nafion to catalyst-containing carbon in the main layer is 30-35 weight percent Nafion (proton conductive material) and 65-70 weight percent catalyst-containing carbon. Desirably, the pH of the slurry composed of carbon and water is about 6-9.
Preferably, the pH is above 6.5 and is about 6.5-9.
Preferably, the average pore size is equal to a radius greater than 5 nanometers. This represents the average pore size of both mesopores and micropores. The current collector supporting the primary (first layer) and the main (second layer) is 0.3 to 0.35 gm / c.
It preferably has a density on the order of m ² .

[0041]

EXAMPLE In this example, a membrane electrode assembly (MEA) was used.
12 were produced. The anode was manufactured by conventional means, and the cathode electrode was manufactured by the improved method of the present invention. In each case, carbon paper was used as the current collector to support the active material component of the electrode. In this example, both Nafion (registered trademark) and Teflon (registered trademark) were used. Nafion® membrane and Nafion® solution are available from Dupont and Solutio, respectively.
Obtained from n Technology. Nafion (registered trademark)
Dupont is a trademark. Teflon is also a trademark of Dupont. Carbon sheet treatment Spectra Corp. Lawrence, MA, 8-11 mil thick, with a density of 0.26 g / cc to 0.7 g / cc.
RaCarb (SC) carbon sheet was obtained. Place carbon paper horizontally on a shelf and mix well with Teflon /
The carbon paper was coated by dipping the carbon paper and shelf in the water mixture for 2 minutes. 1 part (volume) of Teflon 30B solution made by DuPont and 24 parts (volume) of deionized water
Was mixed to produce a Teflon suspension. 1
After drying the sheet at 20 ° C for 15-20 minutes, the carbon paper was sintered in an indirect heating chamber at 320 ° C for 15 minutes and 380 ° C for 30-60 minutes. The Teflon content of the sheet was calculated by weighing the sheet before and after Teflon treatment. The Teflon distribution of the carbon paper was measured using electron microscopy analysis. It was observed that the top of the sheet had a higher Teflon content than the bottom side. After coating the MEA manufactured carbon paper with Teflon, the high Teflon content side was selected for coating the double laminated electrode structure. The primary layer consisted of a barrier layer so that the catalyst slurry did not penetrate the carbon sheet. The slurry for the primary carbon / barrier layer is 5 w / oP in an ultrasonic bath for 2-3 minutes to form a dense slurry.
t, prepared by mixing 10 g deionized water and 13.4 g Nafion solution (5% solution, Solution Technology) with 1 g acetylene black (AB). After applying a layer of AB slurry on top of the Teflon coated carbon sheet using a brush, doctor blade or spray gun, the sheet was dried under a heat lamp at 100 ° C. for 15 minutes. The dry film has a catalyst loading of 0.02
mg / cm ² , Nafion loading 40 w / o and carbon black loading 60 w / o. TEM studies revealed that the primary layer was 10-13 μm thick.

The cathode catalyst is used as a main catalyst layer (second layer)
Nine types of carbon carriers having different characteristics were evaluated for the purpose of supporting them. The support for the anode catalyst was Vulcan XC-72R manufactured by conventional methods.
As the carbon used for the cathode catalyst, both as-received and heat-treated carbons were used. Heat treatment in argon,
Performed at 1000 ° C. for 1 hour. Platinum on carbon (P
t) The catalyst was supported. The catalyst is hexachloroplatinic acid
It was prepared by adding an aqueous solution of (Johnson Matthey) to a carbon / water mixture followed by stirring for 1 hour. Subsequently, Pt (IV) was reduced to a metal state by adding an excess amount of sodium borohydride dropwise to the carbon slurry. After the mixture was stirred for an additional hour, the pH of the solution was adjusted to about 7.0 by adding 1 M sulfuric acid.
Finally filtering the carbon mixture filled with platinum,
Washed thoroughly with water and dried in air at 100 ° C. overnight.
Next, platinum-containing carbon and a 5 w / o Nafion solution
(Solution Technology, Inc., Mendenhall, Pennsylvan
A slurry was prepared by thorough mixing with ia). This catalyst slurry was applied to a carbon sheet previously coated with a primary layer (first layer). The catalyst slurry was applied by brushing and the electrodes were dried at 100 ° C. for 1 hour. The Pt loading was calculated by weighing a sufficiently dried carbon sheet before and after applying each layer. To fabricate the MEA, sandwich a Dow experimental membrane or Nafion 112 membrane between the two electrodes,
MEA 300 ゜ at 500-1000 lb / in 2
It was hot-pressed with F for 1.5 to 2.0 minutes. The MEA evaluation of a membrane electrode assembly with an active electrode area of 25 cm ² was performed using a graphite single cell test fixture (Electrochem, In
c) Installed inside. Using an IBM PC-based data acquisition device and control system, battery potential or current,
Temperature, pressure, gas mass flow and reaction gas humidity
Single cells were operated by a glove-tech fuel cell test station with controlled identification. To regulate the MEA, the cell was operated at 1 A / cm ² with hydrogen / oxygen as reactant at 80 ° C. and 30 psig pressure for 24 hours. Current-voltage curves were recorded at 80 ° C. and various gas pressures using H ₂ / air as reactant. Reactant stoichiometry is 2.5 to 3 with respect to the air, with respect to H ₂ it was 1.2 to 1.5. At the end of each test, a cyclic voltammogram (CV) of the MEA was recorded to determine the electrochemically active surface area of the cathode Pt catalyst as described above. Experimental results Effect of current collector treatment Wet-proofing agen such as Teflon (registered trademark)
After filling in t), a graphite sheet was used as a current collector and a gas diffuser. In addition to changing the Teflon loading in the carbon sheet, the density of the carbon sheet was also changed. Using 20 w / o Pt (supported on Vulcan XC-72R carbon) as a catalyst, Nafion 11
MEA with 2 membranes and Pt loading of 0.28 mg / cm ² / electrode
Was manufactured. FIG. 5 shows the effect of varying the Teflon content of the current collector on fuel cell performance. As the Teflon loading increased, the performance of the battery decreased to lower current densities. Due to the high level of non-conductive Teflon polymer in the matrix, an increase in electrode resistivity is also seen as a side effect. Since Teflon is added to increase the hydrophobicity of the electrode, it seems to be difficult to remove water from the reaction site due to the increased hydrophobicity. This results in electrode flooding, which causes a sharp drop in current at various voltages with increasing Teflon content. With a minimum graphite paper Teflon content of 4 w / o (weight percent), the highest fuel cell performance in this series of experiments (820 mA / Cm ^{2 at} 0.6 V) was obtained. That is, 4% by weight of Teflon and 96% by weight of graphite paper.

FIG. 6 shows the effect of applying a primary carbon layer to a graphite sheet before coating the main catalyst layer. By increasing the density of the main catalyst layer near the membrane interface, the primary layer can easily improve fuel cell performance. Since the catalyst slurry does not penetrate the graphite sheet, the primary carbon layer (first layer)
Plays an important role in using a low-density carbon sheet having excellent performance as described below.

In the PEM fuel cell performance, 0.26 g /
The density effect of the carbon sheet current collector in the range of cc to 0.7 g / cc was studied, and the results are shown in FIG. Due to the limited mass transport, the current density at which the voltage dropped sharply was clearly determined from the density of the paper. Because lower density sheets are more porous, macroporosity (macrop)
water facilitates water removal even at high current densities. 0.7 to 0.26g paper density
As it decreased to / cc, two effects were observed. First, 0.6V before dropping to 0.26g / cc.
Current density at 0.33 g / cc is 0.6
It increased from 2 A / cm ² to 1 A / cm ² . Such an improvement in battery performance at a density of 0.33 g / cc was found, despite increasing from an optimal level of 4 w / o to a Teflon content as high as 8 w / o. The Teflon content increased from 4 to 11.7 w / o as the paper density decreased to incorporate more tephron at lower density from slurries having a constant Teflon concentration in the solution.
As the Teflon content increased, paper density 0.33g / cc
It seems that the maximum was obtained at a current density of 0.6 V. Second, the maximum current density in the linear region of the current-voltage curve (before the sharp drop) is as high as 0.6 A / cm ² to 1.8 A / cm ² at a minimum density of 0.26 g / cc. Increased. Thus, a current collector density of 0.3-0.35 g / cc is considered optimal for cathode applications. Effect of the Nafion Content in the Main Catalyst Layer To ensure good contact between the electrolyte and all catalyst particles, the catalyst layer requires a proton conductive Nafion polymer in its matrix. However, if Nafion is in excess, water will stagnate, which will cause the catalyst sites to load, so the Nafion amount must be optimized. FIG. 8 shows the effect of cathode Nafion content on PEM fuel cell performance. In this series of experiments, 20w / o Pt / Vulc manufactured in-house
An XC-72R catalyst, graphite paper with Teflon content of 19 w / o (10 mil, 0.42 g / cc, Spec)
traCorp). 20w / o to 30w
Increasing the Nafion content to / o (weight percent) greatly improved fuel cell performance, while further increasing decreased cell performance.

In order to explain the effect of the Nafion filling amount, the actual surface area of the platinum catalyst was measured by an electrochemical hydrogen adsorption method, and the results are shown in Table 1. When the Nafion content is less than 30 w / o, it is found that as the Nafion content increases, the substantial Pt surface area also increases. As a result, the accessibility of the catalytic site to the proton-conductive electrolyte (acc.
essibility) is increased. Taking into account the differences in the actual Pt loading, the total Pt loading was used to standardize the Pt surface area, geometric surface area and absolute electrochemical area. From Table 1, 2
It can be seen that increasing the Nafion content from 0 to 0 w / o increases the standardized surface area by 57%, which explains that the fuel cell performance has increased significantly. Even if the Nafion filling amount is increased to 30 w / o or more, the substantial area increases only slightly, so that there was no effect on the fuel cell performance due to the adverse effect of excessive Nafion in the electrode water treatment.

Nafion is a binder, and since a good bond is required between the main layer or the catalyst layer (second layer) and the carbon sheet, more Nafion is used in the primary layer (first layer). It was thought that a filling amount was necessary. Nafion loading in primary layer is 3
When it is reduced to 0 to 35%, the main layer or the catalyst layer (second
Cracks were found in the layer In experiments performed with 30% Nafion in the primary layer, fuel cell performance was also poor. Effect of Carbon Support in Main Catalyst Layer The physico-chemical properties of the carbon support used to disperse the fuel cell catalyst play an important role, especially in battery water treatment at the air cathode. In U.S. Patent Nos. 5,272,017 and 5,316,817, under ambient conditions, ball milled Vulcan XC-7 for anodes is disclosed.
2 and Ketjen black as received for the cathode performed excellently. Physical properties such as total surface area, pore distribution, pore volume, and average pore size were measured to determine the degree of loading driven by the capillary force and the degree of dispersion of the Pt catalyst. Chemical properties, such as surface chemical composition, as measured by slurry pH, determine the hydrophobicity of the pore wall. The semi-hydrophobic region reliably repels water from the electrode matrix and can easily transport reactant gases to the catalyst site. Table 2 lists various physicochemical properties of carbon black that are important for fuel cell electrode performance. Carbon micropores have pore sizes less than 2 nm in diameter, while macropores have pore sizes in the range of 2-50 nm. Acetylene black has the largest proportion of mesopore area, and AX-21 has the smallest proportion of mesopore area. As-received carbon has both acidic and alkaline properties, but all become alkaline upon heat treatment. KetjenBlack and Bla as received
ck Pearls 2000 carbon has the highest pH and Raven 5000 has the lowest pH. In the production of the electrode, the density of the carbon particles and the pore volume allowing gas diffusion are also important. This can be assessed from the volume of 1 gram of carbon black filled with 10 w / o Pt,
As shown in FIG. Acetylene black and Raven 500
0 each had the highest and lowest carbon volumes. Vulcan XC-72R, Ke
tjenBlack, Printex and Black
Pearls 2000 had a similar pore volume.

FIG. 10 shows the fuel cell performance for various as received carbon blacks and heat treated carbon blacks. Although these experiments were not performed at optimal current collector thickness or Teflon loading, these experiments show an important trend that correlates with the hydrophobicity of the carrier. K
It can be seen that B is clearly the highest, but unlike at ambient conditions, high temperature and pressure experiments show that acetylene black, Ketjen Black and Vulcan XC
-72R shows similar performance. Acetylene black, Ketjen Black, Printex and V
Heat treatment of ulcan XC-72R reduced battery performance compared to as-received carbon. Heat-treated Raven 5000 and Black Pearl
The s2000 greatly increased the battery performance by 88% and 43%, respectively. Vulcan XC-72R was subjected to various physical treatments such as ball milling, heat treatment and a combination of ball milling and heat treatment, and the results are shown in FIG. Subjecting Vulcan XC-72R to ball milling or a combination of ball milling / heat treatment reduced cell performance by 40%. For one, the carbon volume is reduced by (6) ball milling, which can limit mass transport.
It would be considered that the average pore radius was reduced (by 30%).

By measuring the actual platinum surface area of Pt catalysts dispersed on various carbons, it is possible to determine why heat treatment reduces the performance of certain carbons and greatly increases the performance of other carbons. Further insight. FIG. 12 shows the effect of carbon type on platinum real surface area, Ketjen Black and AX
-21 showed a maximum platinum surface area of 84 m ² / gm, while AX-21 showed the worst battery performance. This reemphasizes the role of carbon physicochemical properties in improving the utilization of dispersed platinum catalysts. Ketj
Interestingly, the heat treatment reduced the real Pt area of the catalyst dispersed on en Black and Printex by 50%. This indicates that a highly hydrophobic carrier does not disperse the platinum catalyst well, since the platinum solution must penetrate the carbon pores during deposition. This explains why KB, AB and Vulcan as received had superior performance compared to their heat treated form. It can be concluded that a slurry pH in the neutral range of 6-9, especially as received, carbon black with an average pore radius greater than 5 nm is optimal for dispersing Pt catalysts for PEM fuel cell cathode applications (Figure 9 and FIG.
0). Slurry pH is a measurement of the pH of a carbon slurry in water.

Since acetylene black (AB) had an optimum pH for the semi-hydrophobic carrier, the pH of the primary carbon layer (first layer) was not changed. It is unlikely that the optimal pH range for the primary layer (first layer) is significantly different from the main or catalyst layer (second layer).

The optimum pore radius of the primary layer (first layer) may be the same as that of the catalyst layer, but need not be different. However, the volume of carbon per unit mass may be important. AB has the highest volume per gram because AB is the lowest density (FIG. 9). Thus, the AB will mechanically close the holes in the carbon sheet without significantly impeding the transport of gas throughout the holes in the primary layer. On this basis, these carbon particles are preferably characterized by a volume per gram of at least about 10 cm ³ / gm. This corresponds to a density of about 0.1 gm / cm ³ or less for the carbon particles in the primary layer.

The catalyst can optionally be included in the primary layer (first layer), but is not essential as the reaction zone is unlikely to extend beyond the main or catalyst layer (second layer) . However, the addition of traces of catalyst (platinum) improves the conductivity of the matrix and promotes battery performance. A very low loading of 0.02 mg / cm ² is sufficient, but this amount is reasonable,
Further increasing the loading does not seem to be beneficial. A range on the order of 0 to 0.15 mg / cm ² or less is considered reasonable.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

In summary, the present invention provides a membrane-electrode assembly (ME) comprising a membrane sandwiched between two carbon sheet current collectors carrying a catalyst layer for a fuel cell reaction.
The essential components of the PEM fuel cell containing A) are improved. The features described herein enhance the rate of transporting oxygen to the reaction site at the membrane / electrode interface and improve the removal of product water. This carefully optimizes the design and structure of the air electrode (cathode); graphite paper density and its Teflon content; Nafion loading in the reaction layer; and the slurry pH and pore distribution of the carbon support used to disperse the catalyst. This is achieved by: These features improve catalyst dispersion, gas transport to the catalyst layer, and water treatment.

Nafion in the electrode acts as a binder in the catalyst layer, as good as the proton-conductive electrolyte. Carbon supports have been studied early for batteries operated at room temperature and near ambient pressure. Ket of cathode
Jen Black and anode ball milled Vulcan XC-72R have been found to be the best carbon black supports for optimal water treatment and dispersion of platinum catalysts (US Pat. Nos. 5,272,017 and 5). , 316,817).

Prior to the improvement described in US Pat. Nos. 5,272,017 and 5,316,817, a method of manufacturing a membrane electrode assembly (MEA) was to coat the membrane with a platinum-treated carbon slurry. And then applying a carbon sheet to the membrane as a current collector. This had the disadvantage that it was primarily suitable for high equivalent weight thick films such as Nafion 117. US Patent 5,27
The methods of 2,017 and 5,316,817 are:
This involved applying the catalyst-containing carbon slurry directly to the carbon sheet, followed by hot pressing the electrodes on the membrane.
The approach of the present invention uses a multilayer electrode structure that can be readily applied to any type of carbon sheet or proton exchange membrane for mass production and for gas diffusion linings.

The multilayer cathode structure has a very small amount (0.02
mg / cm ² ) of Pt and a suitably treated carbon black main primary catalyst layer filled with 20 w / o Pt.
The primary layer improves the coating performance of the main catalyst layer and facilitates improving battery performance by localizing the layer near the membrane interface. The performance of the main catalyst layer was optimized with a carbon support that was hydrophobic enough to repel water from the electrode matrix, but sufficient to disperse the Pt catalyst with high catalyst utilization. The filling of the Nafion polymer into the main catalyst layer and the Teflon polymer into the carbon sheet current collector were also optimized for higher gas sparging and catalyst utilization. 0.3-0.3
Carbon sheets with densities in the range of 5 g / cc and a Teflon content of less than 5 w / o have been found to be optimal for the current collector. With a cathode Nafion content of 30-35 w / o, acceptable Pt utilization could be achieved while keeping electrode loading to a minimum. Among various carbon materials with a wide range of properties evaluated as cathode catalyst supports, carbon with an average pore radius of greater than 5 nm and a slurry pH in the neutral range of 6-9 was found to be optimal for cathode applications. .

The excellent performance of the hydrogen / air cell presented herein is achieved by various manufacturing and composition parameters such as cathode Nafion content, carbon sheet density and Teflon content, and is useful for dispersing the catalyst. All the physicochemical properties of the carbon support used were optimized. The effectiveness of the present invention is evident from the test results described herein.

Although the invention has been described with respect to particular embodiments, it is not limited to the above description, but only by the claims appended hereto.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined in the following claims.

[Brief description of the drawings]

FIG. 1 is a schematic illustration of an unassembled electrochemical fuel cell having an electrode according to the present invention and a combined membrane and electrode assembly.

FIG. 2 is a pictorial cross-sectional view of the membrane electrode assembly of the present invention.

FIG. 3 is another pictorial illustration of a membrane electrode assembly having a graphite sheet.

FIG. 4 is an enlarged view showing carbon particles carrying catalyst particles mixed with a proton conductive material.

FIG. 5 shows the effect of current collector Teflon content in a PEM fuel cell operated at 80 ° C., air / H ₂ , 3 / 1.2 stoichiometry, 30 psig. 20w / oPt
Vu, 10 mil SC, 0.5 g / cc, Nafion 112 film, Pt loading = 0.28 mg / cm ² / electrode.

FIG. 6 shows the effect of using a primary carbon / catalyst layer on PEM fuel cell performance. 20 w / o PtV for main layer and primary layer respectively
u and 5 w / o PtAB were used. Nafion 112
Membrane; Pt loading; 0.35 mg / cm ² / electrode; air /
Hydrogen, 80 ° C., 30 psig; 3 / 1.2 stoichiometric.

FIG. 7 shows the effect of current collector density on a PEM fuel cell operated at 80 ° C., air / H ₂ , 3 / 1.4 stoichiometry, 30 psig. 20 w / o PtVu, Nafion 112 film, Pt loading = 0.3 mg / cm ² / electrode.

FIG. 8: 80 ° C., air / H ₂ , 3 / 1.5 stoichiometry, 55/30 psig, 20 w / o PtVu, 1
7 shows the effect of cathode Nafion content on PEM fuel cell performance when operated with 0 mil SC, 0.42 g / cc, 19 w / o Teflon, Dow membrane, Pt loading = 0.45 mg / cm ² / cell.

FIG. 9 shows the volume of 1 gram of 10 w / o carbon-supported Pt catalyst.

FIG. 10 shows 0.5 V, 80 ° C., air / H ₂ ,
30 psig, 3 / 1.5 stoichiometry, 10 w / oPt
/ Carbon, 10 mil SC, 0.42 g / cc, 2
5w / o Teflon, Dow film, Pt loading = 0.11 ±
9 shows the effect of carbon type on PEM fuel cell performance when operated at 0.02 mg / cm ² / electrode.

FIG. 11 shows 80 ° C., air / H ₂ , 30 psi.
g, 3 / 1.5 stoichiometry, 10 w / o PtVu, 10
Vulcan XC in PEM battery performance when operated on mil SC, 0.42 g / cc, 25 w / o Teflon, Dow membrane, Pt = 0.23 mg / cm ² / battery
The effect of the -72R treatment is shown.

FIG. 12 shows the effect of carbon type on platinum electrochemical surface area.

FIG. 13 shows a plot of cell current density at 0.5 volts versus pH of the carbon slurry used to disperse the Pt catalyst. The experimental conditions are the same as in FIG.

FIG. 14 shows a plot of cell current density at 0.5 volts versus average pore radius of the cathode carbon support. The experimental conditions are the same as in FIG.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Joseph Mikheil, Windsor Court, Stirling Heights, 48313, Michigan, USA 12702

Claims (20)

[Claims] 1. A current collector sheet, a first electrode layer, and a second electrode layer.
An electrode structure comprising: an electrode layer, wherein the first electrode layer is between the current collector sheet and the second electrode layer;
The first layer includes a first group of carbon particles and the second layer includes a second group of carbon particles; wherein the first layer does not include a catalyst or comprises a finely divided catalyst particle. The second layer comprises a second group of micronized catalyst particles; provided that the weight ratio of catalyst particles to carbon particles in the first layer is the weight ratio of the second layer to that of the second layer. Said electrode structure. 2. Each group of carbon particles includes a plurality of carbon particles having an inner surface and an outer surface defining a number of pores between and within the carbon particles, and a plurality of carbon particles inside and outside the carbon particles. The electrode structure according to claim 1, wherein the electrode structure comprises the micronized catalyst particles supported on a surface. 3. The method according to claim 2, wherein the first layer does not include a catalyst, and the second layer does not include a catalyst.
The second group of carbon particles having internal and external surfaces defining a plurality of pores between and within the carbon particles, the micronization carried on the internal and external surfaces of the carbon particles. The electrode structure according to claim 1, comprising a second group of catalyst particles, and a proton conductive material intermingled with the carbon particles and the catalyst particles. 4. The electrode structure according to claim 1, wherein said first group of carbon particles has a density of less than or equal to 0.1 grams per cubic centimeter. 5. The electrode structure according to claim 1, wherein said second group of carbon particles is characterized by a pH in the range of about 6 to about 9. 6. The electrode structure according to claim 1, wherein each group of carbon particles is characterized by a pH in the range of about 6 to about 9. 7. The electrode structure according to claim 1, wherein said second group of carbon particles is characterized by an average pore radius of more than 5 nanometers. 8. The electrode structure according to claim 1, wherein each of said layers further comprises a proton conductive material mixed with carbon particles and catalyst particles. 9. The catalyst loading of the second layer is less than about 0.30 mg / cm ^{2 of} electrode surface area, and the catalyst loading of the first layer is less than the loading of the second layer. Item 2. The electrode structure according to Item 1. 10. The second layer comprises the catalyst particles and the carbon particles in a weight ratio of about 20:80; the proton conductive material comprises 30-35 weight percent of the second layer; The electrode structure according to claim 1, wherein the catalyst and the carbon particles constitute a residue. 11. The method according to claim 1, wherein the current collector includes a carbon sheet impregnated with Teflon (registered trademark), and Teflon constitutes about 5 parts or less based on 100 parts by weight of the carbon sheet and Teflon. The electrode structure as described in the above. 12. The carbon sheet before impregnation,
The electrode structure of claim 11, wherein the electrode structure has a thickness of 12 mils and a density of about 0.3 to about 0.35 g / cc. 13. The electrode structure according to claim 1, wherein the first group and the second group of the carbon particles are of the same type. 14. The first group and the second group of carbon particles,
The electrode structure according to claim 1, wherein the electrode structure can be identified by one or more of pH, pore size, particle size, and BET surface area. 15. An electrode structure including a current collector sheet, a first electrode layer, and a second electrode layer, wherein the first electrode layer includes the current collector sheet and the second electrode layer. And the first layer and the second layer include a first group and a second group, respectively, of catalyst-containing carbon particles, wherein the catalyst-containing carbon particles carry micronized catalyst particles. Wherein the weight ratio of catalyst particles to carbon particles in the first layer is less than the weight ratio of the second layer. 16. A (a) providing a current collector sheet; (b) a proton conductive material, a first group of carbon particles;
And optionally forming a first layer comprising catalyst particles on the sheet; and (c) a proton conductive material, a second group of carbon particles;
And a second layer including catalyst particles is formed on the first layer, wherein the weight of the catalyst particles with respect to the carbon particles of the second layer is greater than the weight of the first layer. The method of manufacturing the structure. 17. forming a first mixture of a proton conductive material, a first group of carbon particles, and a first group of micronized catalyst particles supported on and within the carbon particles; 17. The method of claim 16, wherein step (b) is performed by applying the first mixture to a surface of a current collector and forming a first film from the mixture. 18. A method for forming a first mixture of a first group of a proton conductive material and carbon particles; and then applying the first mixture to a surface of a current collector to form a first film from the mixture. 17. The method of claim 16, wherein step (b) is performed by performing. 19. The electrode structure according to claim 16, wherein the first group of carbon particles has a density of less than or equal to 0.1 grams per cubic centimeter. 20. forming a second mixture of a proton conductive material, a second group of carbon particles, and a second group of micronized catalyst particles supported on and within the carbon particles;
17. The method of claim 16, wherein step (c) is then performed by applying a second mixture over the first layer.